Random access method and apparatus therefor

ABSTRACT

A disclosure of the present specification provides a random access method for an unlicensed band. The method comprises the steps of: transmitting, by a user equipment (UE), a random access preamble in a first cell of the unlicensed band; and receiving, by the UE, a random access response in a second cell, wherein the second cell may be a first cell, a cell of the unlicensed band, included in a cell group to which the first cell belongs, or a cell of the unlicensed band, included in a timing advance group (TAG) within the cell group, to which the first cell belongs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 15/531,277 filed on May 26, 2017, which is the National Phaseof PCT International Application No. PCT/KR2015/012833 filed on Nov. 27,2015, which claims the priority benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application No. 62/085,319 filed on Nov. 27, 2014, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a random access method in a wireless communicationsystem, and an apparatus using the method.

Discussion of the Related Art

With the explosive increase in mobile data traffic in recent years, aservice provider has utilized a wireless local area network (WLAN) todistribute the data traffic. Since the WLAN uses an unlicensed band, theservice provider can address a demand for a significant amount of datawithout the cost of an additional frequency. However, there is a problemin that an interference phenomenon becomes serious due to a competitiveWLAN installation between the providers, quality of service (QoS) cannotbe guaranteed when there are many users, and mobility cannot besupported. As one of methods for compensating this, a long termevolution (LTE) service in the unlicensed band is emerged.

LTE in unlicensed spectrum (LTE-U) or licensed-assisted access using LTE(LAA) is a technique in which an LTE licensed band is used as an anchorto combine a licensed band and an unlicensed band by the use of carrieraggregation (CA). A user equipment (UE) first accesses a network in thelicensed band. A base station (BS) may offload traffic of the licensedband to the unlicensed band by combining the licensed band and theunlicensed band according to a situation.

The LTE-U may extend an advantage of LTE to the unlicensed band toprovide improved mobility, security, and communication quality, and mayincrease a throughput since the LTE has higher frequency efficiency thanthe legacy radio access technique.

Unlike the licensed band in which exclusive utilization is guaranteed,the unlicensed band is shared with various radio access techniques suchas the WLAN. Therefore, each communication node acquires a channel to beused in the unlicensed band in a contention-based manner, and this iscalled a carrier sense multiple access with collision avoidance(CSMA/CA). Each communication node must perform channel sensing beforetransmitting a signal to confirm whether a channel is idle, and this iscalled clear channel assessment (CCA).

A random access is a procedure for the UE to acquire uplinksynchronization with a serving cell. Since the conventional randomaccess procedure is designed under assuming that operating in a licensedband, it may be inefficient in the unlicensed band considering aninterference between communication nodes.

SUMMARY OF THE INVENTION

The present invention provides a random access method operating in aunlicensed band and an apparatus using the method.

In an aspect, a random access method for an unlicensed band is provided.The method includes transmitting, by a user equipment (UE), a randomaccess preamble in a first cell of the unlicensed band, and receiving,by the UE, a random access response in a second cell. The second cell isa cell of the unlicensed band, which is included in a cell group towhich the first cell belongs, or a cell of the unlicensed band, which isincluded in a timing advance group (TAG) within the cell group to whichthe first cell belongs.

The second cell may be a cell of the unlicensed band in which downlink(DL) transmission for the UE is scheduled most quickly after the randomaccess preamble is transmitted in the cell group.

The method may further include transmitting, by the UE, a scheduledmessage in the third cell according to an uplink (UL) grant in therandom access response.

The third cell may be a cell of a licensed band in the cell group towhich the first cell belongs.

The random access response may comprise information regarding the thirdcell.

The method may further include adjusting, by the UE, a UL transmissiontiming for a target cell on the basis of a time alignment command (TAC)comprised in the random access response. The random access response maycomprise information regarding the target cell.

In another aspect, an apparatus for performing a random access by usingan unlicensed band is provided. The apparatus includes a radio frequency(RF) unit, and a processor for controlling the RF unit and configured tocontrol the RF unit to transmit a random access preamble in a first cellof the unlicensed band and control the RF unit to receive a randomaccess response in a second cell. The second cell is a cell of theunlicensed band, which is included in a cell group to which the firstcell belongs, or a cell of the unlicensed band, which is included in atiming advance group (TAG) within the cell group to which the first cellbelongs.

A random access may be performed by considering interference withvarious communication nodes in an unlicensed band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a long term evolution (LTE) system using anunlicensed band.

FIG. 2 shows an example of a cell group.

FIG. 3 shows an example of a random access procedure according to theconventional technique.

FIG. 4 shows an example of a random access response according to theconventional technique.

FIG. 5 shows a random access method according to an embodiment of thepresent invention.

FIG. 6 shows a random access method according to another embodiment ofthe present invention.

FIG. 7 shows a random access preamble according to an embodiment of thepresent invention.

FIG. 8 is a block diagram showing a wireless communication system forimplementing the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention is applied based on a 3rd generationpartnership project (3GPP) long term evolution (LTE) or a 3GPPLTE-Advanced (LTE-A). This is for exemplary purposes only, and thus thepresent invention is applicable to various communication systems. In thefollowing description, LTE and/or LTE-A are collectively referred to asLTE.

Further, a base station (BS) used hereinafter is generally a fixedstation that communicates with a wireless device, and may be referred toas another terminology, such as an evolved-NodeB (eNodeB), anevolved-NodeB (eNB), a base transceiver system (BTS), an access point,etc.

In addition, a user equipment (UE) may be fixed or mobile, and may bereferred to as another terminology, such as a wireless device, a mobilestation (MS), a user terminal (UT), a subscriber station (SS), a mobileterminal (MT), or the like.

The UE may be served by a plurality of serving cells. The serving cellmay be classified into a primary cell (PCell) and a secondary cell(SCell). The PCell operates at a primary frequency, and is a celldesignated as the PCell when an initial network entry process isperformed or when a network re-entry process starts or in a handoverprocess. The PCell is also called a reference cell. The SCell operatesat a secondary frequency. The SCell may be configured after a radioresource control (RRC) connection is established, and may be used toprovide an additional radio resource. At least one PCell is configuredalways. The SCell may be added, modified, or released by usinghigher-layer signaling (e.g., an RRC message).

FIG. 1 shows an example of an LTE system using an unlicensed band.

A UE 10 establishes a connection with a 1^(st) BS 20, and thereafterreceives a service through a licensed band. In addition, for trafficoffloading, the UE 10 may receive a service through an unlicensed bandwith respect to a 2^(nd) BS 30.

The 1^(st) BS 20 is a BS supporting the LTE system, whereas the 2^(nd)BS 30 may also support other communication protocols such as a wirelesslocal area network (WLAN) other than LTE. The 1^(st) BS 20 and the2^(nd) BS 30 may be coupled based on a carrier aggregation (CA), and aspecific cell of the 1^(st) BS 20 may be designated as a PCell.Alternatively, the 1^(st) BS 20 and the 2^(nd) BS 30 may be coupledbased on a dual connectivity, and a specific cell of the 1^(st) BS 20may be designated as the PCell.

In general, the 1^(st) BS 20 having the PCell has wider coverage thanthe 2^(nd) BS 30. Therefore, the 1^(st) BS 20 may be called a macrocell. The 2^(nd) BS 30 may be called a small cell, a femto cell, or amicro cell. The 1^(st) BS 20 may operate the PCell and zero or at leastone SCell. The 2^(nd) BS 30 may operate at least one SCell. In addition,the SCell may be activated or deactivated by an indication of the PCell.The above description is for exemplary purposes only, and thus the1^(st) BS 20 may correspond to the PCell, and the 2^(nd) BS 30 maycorrespond to the SCell, so that the cell can be managed by one BS.

The licensed band is a band in which an exclusive use is guaranteed to aspecific communication protocol or a specific provider.

The unlicensed band is a band in which various communication protocolscoexist and a shared use is guaranteed. The unlicensed band may include2.5 GHz and/or 5 GHz band used in a WLAN, but the present invention isnot limited thereto.

In general, it is assumed that a channel is ensured in the unlicensedband through contention between respective communication nodes.Therefore, for communication in the unlicensed band, it is required toperform channel sensing and confirm that signal transmission is notachieved by other communication nodes. As such, if it is determined thatsignal transmission is not achieved by other communication nodes, thiscase is defined as confirmation of clear channel assessment (CCA).

Since transmission of downlink (DL) from a BS or uplink (UL) from a UEmay not always be guaranteed in the unlicensed band, the UE operating inthe unlicensed band may always maintain an access to a cell operating ina licensed band to reliably control a mobility or radio resourcemanagement (RRM) function or the like.

The LBT must be performed preferentially in order for a BS or UE of anLTE system to have access to a channel in the unlicensed band. Further,when the BS or UE of the LTE system transmits a signal, othercommunication nodes such as the WLAN or the like must also perform theLBT so that interference does not occur. For example, in the WLAN, a CCAthreshold is defined as −62 dBm as to a non-WLAN signal and is definedas −82 dBm as to a WLAN signal. This means that, if an LTE signal isreceived with power less than or equal to −62 dBm, other communicationnodes do not transmit signals so that the interference does not occur.

Hereinafter, when it is said that ‘LBT is performed’ or ‘CCA isperformed’, it implies that whether a channel is idle or is used byanother node is confirmed first and thereafter the channel is accessed.

In addition, hereinafter, a cell operating in a licensed band isreferred to as a licensed band cell, and a cell operating in anunlicensed band is referred to as an unlicensed band cell.

FIG. 2 shows an example of a cell group.

A primary cell group (PCG) is a serving cell group including a primarycell (PCell) and zero or more secondary cells (SCells). A secondary cellgroup (SCG) is a serving cell group including a primary secondary cell(PSCell) and zero or more secondary cells (SCells). The PSCell may be anSCell for performing a random access in the SCG. It may be divided intoone or more timing advance groups (TAGs) in the serving cell group. TheTAG is a cell group which is configured by a radio resource control(RRC) and which uses the same timing advance (TA).

The PCG includes a TAG1 and a TAG2. The TAG1 includes a primary cell(PCell) and a first secondary cell (SCell1), and the TAG2 includes asecond secondary cell (SCell2) and a third secondary cell (SCell3). TheSCG includes a TAG3 and a TAG4. The TAG3 includes a PSCell and a fourthsecondary cell (SCell4), and the TAG4 includes a fifth secondary cell(SCell5) and a sixth secondary cell (SCell6). The number of TAGs andcells included in each TAG are for exemplary purposes only.

Although the PCell operates in the licensed band, at least any one ofthe PSCell and the SCell1 to SCell6 may operate in the unlicensed band.An unlicensed band cell or a licensed band cell may be configured as theSCell through RRC signaling in the PCell, and information regarding towhich TAG the S Cell belongs may also be provided to a UE.

FIG. 3 shows an example of a random access procedure according to theconventional technique.

The random access procedure is used by a UE to obtain uplinksynchronization with a BS or to allocate an uplink radio resource.

The UE receives a root index and a physical random access channel(PRACH) configuration index from the BS.

The root index is a logical index used by the UE to generate 64candidate random access preambles (RAPs). The RAP is a signaltransmitted by the UE in an area predetermined for UL synchronization.The RAP may be referred to as a synchronization signal in a sense thatit is a signal for requesting UL synchronization.

The PRACH configuration index indicates a specific subframe and preambleformat capable of transmitting the RAP. The RAP is limited to a specifictime and frequency resource for each cell.

The UE selects any RAP, and transmits the selected RAP to the BS (S110).More specifically, the UE selects one of the 64 candidate RAPs. The UEselects a corresponding subframe by using the PRACH configuration index.In addition, the UE transmits the selected RAP through the selectedsubframe.

Upon receiving the RAP, the BS transmits a random access response (RAR)to the UE (S120). The RAR is detected in two steps. First, the UEdetects a physical downlink control channel (PDCCH) masked with a randomaccess-radio network temporary identifier (RA-RNTI). Next, the UEreceives the RAR included in a medium access control (MAC) protocol dataunit (PDU) through a physical downlink shared channel (PDSCH) indicatedby the detected PDCCH.

FIG. 4 shows an example of a random access response according to theconventional technique.

The random access response may include a time alignment command (TAC), aUL grant, and a temporary cell-RNTI (C-RNTI).

The TAC is information indicating a TA value which is sent by a BS to aUE to maintain a UL time alignment (TA). The UE updates UL transmissiontiming by using the TA value. A time alignment timer is started orrestarted when the UE updates the TA.

The UL grant includes a UL resource allocation and transmit powercommand (TPC) used in message transmission. The TPC is used to determinetransmit power for a scheduled PUSCH.

Returning to FIG. 3, the UE transmits to the BS a scheduled messageaccording to the UL grant included in the random access response (S130).

The random access procedure based on the conventional techniqueconsiders only one serving cell. Therefore, a TA for one serving cell inwhich a random access preamble is exchanged with the random accessresponse may be obtained, and UL synchronization may be adjusted.

However, a plurality of serving cells may be configured to one UE, andthe plurality of serving cells may operate in different TAGs ordifferent bands (e.g., a licensed band or an unlicensed band).

Hereinafter, a method of performing a random access procedure in anunlicensed band cell is proposed. The unlicensed band cell may be anSCell or an independent cell.

FIG. 5 shows a random access method according to an embodiment of thepresent invention.

A UE may transmit a random access preamble in a first cell of anunlicensed band (S210).

The UE receives a random access response in a second cell (S220). Therandom access response may include information regarding UL transmissiontiming and/or transmit power of the UE. In addition, the random accessresponse may include information regarding a cell or cell group when theUE transmits a scheduled message.

The second cell in which the UE receives the random access response(RAR) may be defined as follows.

1) The second cell may be a licensed band cell included in a cell groupto which the first cell for transmitting the random access preamblebelongs. More specifically, the licensed band cell may be a PCell or aPSCell.

2) The random access preamble may be transmitted at the request of aspecific cell. The second cell may be a cell for requesting transmissionof the random access preamble or a cell included in a cell group towhich the cell for requesting transmission of the random access preamblebelongs.

3) The second cell may be an unlicensed band cell included in the firstcell in which the random access preamble is transmitted, or a cell groupto which the first cell belongs or a TAG to which the first cellbelongs.

The second cell may be an unlicensed band cell in which DL transmissionfor a corresponding UE is scheduled most quickly after the random accesspreamble is transmitted.

4) The second cell may be determined on the basis of information such asa cell ID, sequence, timing, frequency resource, or the like of thefirst cell in which the random access preamble is transmitted.

The UE adjusts UL transmission timing and/or transmit power on the basisof the information included in the received random access response(S230).

The UE may adjust the UL transmission timing and/or the transmit poweras follows.

1) The UE may adjust the UL transmission timing and/or transmit powerfor the first cell which has transmitted the random access preamble andan unlicensed band cell belonging to the same TAG of the first cell.

2) The UE may adjust the UL transmission timing and/or transmit powerfor a cell or TAG in which a scheduled message is transmitted.

The random access response may include information regarding a targetcell for adjusting the UL transmission timing and/or the transmit power.The UE may adjust the UL transmission timing and/or the transmit poweraccording to the information regarding the target cell.

The UE may transmit a scheduled message according to a UL grant includedin the random access response in a third cell (S240). The third cell maybe one of the licensed band cell and the unlicensed band cell.

The third cell in which the UE transmits the scheduled message may bedefined as follows.

1) If the random access preamble is transmitted in an unlicensed bandcell belonging to a master cell group (MCG), the third cell may be alicensed band cell belonging to the MCG. More particularly, the thirdcell may be a primary cell (PCell) in the MCG.

2) If the random access preamble is transmitted in an unlicensed bandcell belonging to an SCG, the third cell may be a licensed band cellincluded in the SCG. More particularly, the third cell may be a PSCellin the SCG.

3) The third cell may be a licensed band cell included in a cell groupto which the first cell belongs or a TAG to which the first cellbelongs.

4) The third cell may be an unlicensed band cell included in a cellgroup to which the first cell belongs or a TAG to which the first cellbelongs.

5) The third cell may be an unlicensed band cell which is scheduled mostquickly after receiving a random access response among a plurality ofunlicensed band cells, and in which UL transmission is possible as aresult of confirming a channel state.

6) The third cell may be a licensed band cell in a TAG to which thefirst cell for transmitting the random access preamble belongs. If thelicensed band cell is not included in the TAG to which the first cellbelongs, the third cell may be the unlicensed band cell in the TAG.

7) The third cell may be one of a plurality of licensed band cells orunlicensed band cells determined on the basis of information such as acell ID, sequence, timing, frequency resource, or the like of the firstcell in which the random access preamble is transmitted.

8) If information regarding a cell or cell group in which the scheduledmessage is transmitted is included in the random access response, thethird cell may be determined on the basis of the information.

FIG. 6 shows a random access method according to another embodiment ofthe present invention.

Assume that a BS manages a first cell, a second cell, and a third cell.The first cell, the second cell, and the third cell may be the same cellor may be different cells.

If a channel is idle after performing CCA, a UE transmits a randomaccess preamble 610 in the first cell of an unlicensed band. The UEreceives a random access response 620 in the second cell. Then, the UEtransmits a scheduled message 630 in the third cell.

FIG. 7 shows a random access preamble according to an embodiment of thepresent invention.

A UE may transmit a legacy short training field (L-STF) 601, a legacylong training field (L-LTF) 602, and a legacy signal field (L-SIG) 603before transmitting the random access preamble 610.

The L-STF 601 may be utilized in carrier sensing for detecting that asignal exists in a channel currently in use, automatic gain control foradjusting a radio signal which is input to an antenna to an operatingarea of an analog circuit and an analog-to-digital converter, and coarsecarrier frequency offset correction.

The L-LTF 602 may be used for fine carrier frequency offset correctionand symbol synchronization, and may be used for channel responseestimation for demodulation of the L-SIG 603.

The L-SIG 603 is generated as a bit sequence having informationindicating a transmission time to prevent another communication nodefrom accessing a corresponding channel during a corresponding randomaccess procedure is performed. More specifically, the L-SIG 603 mayindicate a transmission time (T1 of FIG. 6) corresponding to a sum of atime required to transmit a random access preamble and a time requiredto receive a random access response. Alternatively, the L-SIG 603 mayindicate a transmission time (T2 of FIG. 6) corresponding to a sum ofthe time required to transmit the random access preamble, the timerequired to receive the random access response, and a time required totransmit a scheduled message.

FIG. 8 is a block diagram showing a wireless communication system forimplementing the present invention.

A BS 200 includes a processor 201, a memory 202, and a radio frequency(RF) unit 203. The memory 202 is coupled to the processor 201, andstores a variety of information for driving the processor 201. The RFunit 203 is coupled to the processor 201, and transmits and/or receivesa radio signal. The processor 201 implements the proposed functions,procedures, and/or methods. In the aforementioned embodiment, anoperation of the BS may be implemented by the processor 201.

A UE 100 includes a processor 101, a memory 102, and an RF unit 103. Thememory 102 is coupled to the processor 101, and stores a variety ofinformation for driving the processor 101. The RF unit 103 is coupled tothe processor 101, and transmits and/or receives a radio signal. Theprocessor 101 implements the proposed functions, procedures, and/ormethods.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps.

Furthermore, those skilled in the art will understand that the stepsshown in the flowcharts are not exclusive and may include other steps orone or more steps of the flowcharts may be deleted without affecting thescope of the present invention.

1. A random access method for an unlicensed band, the method comprising:transmitting, by a user equipment (UE), a random access preamble in afirst cell of the unlicensed band; and receiving, by the UE, a randomaccess response in a second cell of the unlicensed band, wherein thesecond cell is a specific cell among a plurality of cells in theunlicensed band, wherein the second cell of the unlicensed band isdetermined after the random access preamble is transmitted, and whereinthe determination of the second cell is related to an operation oflisten before talk (LBT) or a clear channel assessment (CCA).
 2. Themethod of claim 1, wherein the second cell is the specific cell amongthe plurality of cells in the unlicensed band in which the operation ofLBT or CCA was performed successfully.
 3. The method of claim 1, whereinthe operation of LBT or CCA is performed in order to prevent aninterference of signals in the unlicensed band.
 4. The method of claim1, wherein the second cell is the specific cell of the unlicensed band,which is included in a cell group to which the first cell belongs, or acell of the unlicensed band, which is included in a timing advance group(TAG) within the cell group to which the first cell belongs.
 5. Themethod of claim 1, wherein the second cell is the specific cell of theunlicensed band in which downlink (DL) transmission for the UE isscheduled most quickly after the random access preamble is transmittedin the cell group.
 6. The method of claim 1, further comprising:transmitting, by the UE, a scheduled message in a third cell accordingto an uplink (UL) grant in the random access response.
 7. The method ofclaim 6, wherein the random access response comprises informationregarding the third cell.
 8. The method of claim 1, further comprising:adjusting, by the UE, an uplink (UL) transmission timing for a targetcell on the basis of a time alignment command (TAC) comprised in therandom access response, wherein the random access response comprisesinformation regarding the target cell.
 9. An apparatus for performing arandom access by using an unlicensed band, the apparatus comprising: aradio frequency (RF) unit, and a processor for controlling the RF unitand configured to: control the RF unit to transmit a random accesspreamble in a first cell of the unlicensed band; and control the RF unitto receive a random access response in a second cell of the unlicensedband, wherein the second cell is a specific cell among a plurality ofcells in the unlicensed band, wherein the second cell of the unlicensedband is determined after the random access preamble is transmitted, andwherein the determination of the second cell is related to an operationof listen before talk (LBT) or a clear channel assessment (CCA).
 10. Theapparatus of claim 9, wherein the second cell is the specific cell amongthe plurality of cells in the unlicensed band in which the operation ofLBT or CCA was performed successfully.
 11. The apparatus of claim 9,wherein the operation of LBT or CCA is performed in order to prevent aninterference of signals in the unlicensed band.
 12. The apparatus ofclaim 9, wherein the second cell is the specific cell of the unlicensedband, which is included in a cell group to which the first cell belongs,or a cell of the unlicensed band, which is included in a timing advancegroup (TAG) within the cell group to which the first cell belongs. 13.The apparatus of claim 9, wherein the second cell is the specific cellof the unlicensed band in which downlink (DL) transmission for the UE isscheduled most quickly after the random access preamble is transmittedin the cell group.
 14. The apparatus of claim 9, wherein the processoris configured to control the RF unit to transmit a scheduled message ina third cell according to an uplink (UL) grant in the random accessresponse.
 15. The apparatus of claim 14, wherein the random accessresponse comprises information regarding the third cell.
 16. Theapparatus of claim 9, wherein the processor is configured to control theRF unit to adjust a UL transmission timing for a target cell on thebasis of a time alignment command (TAC) comprised in the random accessresponse, and wherein the random access response comprises informationregarding the target cell.